browser compatibility

am working on a big project and the project is almost 75% finished , but I haven't taught about browser compaiblity a head of time and now I am suffering the consequences. on internet explorer,older version of firefox and mobile phones it ain't working properly.in this browsers the whole page is distorted and elements overlap with elements, and this give the site a horrible and disgusting look.

the only solution i can think of right now is working on the css file it is kind of tiresome and time consuming. if any of u has any easier way, recommend me.